The present invention relates to a sensor for detecting the input air-fuel ratio of an internal combustion engine from the oxygen and unburned gas composition in the exhaust gas of the engine.
A known air-fuel ratio sensor arrangement, such as disclosed, for example, in Japanese Patent Application Laid-Open No. 60-128349 (1985), Japanese Patent Application Laid-Open No. 61-138155 (1986), Japanese Patent Application Laid-Open No. 62-265560 (1987), Japanese Patent Application Laid-Open No. 62-198750 (1987), Japanese Patent Application Laid-Open No. 4-134152 (1992) and so on, comprises a concentration cell having an atmosphere air side electrode which is exposed directly to atmospheric air, and an exhaust gas side electrode which is covered with an exhaust gas diffusion rate control member, the two electrodes being provided on opposite sides of a solid-state electrolyte having oxygen ion conductivity. Ion-pumping (hereinafter referred to as "pumping") of oxygen molecules is performed between the atmospheric air side electrode and the exhaust gas side electrode such that the electromotive force of the concentration cell due to the ratio of oxygen partial pressure induced between the atmospheric air side electrode and the exhaust gas side electrode is kept constant, and the oxygen concentration in the exhaust gas is detected from the pumping current.
Air-fuel ratio sensors of this type have an exhaust gas pressure dependence, as discussed in T. Kamo, et al: Lean Mixture Sensor, SAE Paper 850380, (1985). According to this reference, the exhaust gas pressure dependence is classified based on the shape of the diffusion rate control member as follows:
1. In the case of a simple pore film: Diffusion is primarily molecular diffusion (exhaust pressure dependence is small); PA1 2. In a case of a porous film: Diffusion is primarily Knudsen diffusion (exhaust pressure and pumping current are in a proportional relationship). PA1 where h is the elevation above sea level (m); T is the average temperature (.degree.C.); Po is standard atmospheric pressure at sea level; and Ph is the atmospheric pressure at altitude h.
Actually, in both of the above cases, the exhaust gas pressure dependence of the output from the sensor is large enough to require correction.
Exhaust gas pressure dependence can be divided into two components: one being governed by engine parameters, and another governed by atmospheric pressure. The former may be estimated from engine parameters, while the latter dependence cannot be corrected unless atmospheric pressure is measured. The atmospheric pressure dependence is such that the air-fuel ratio sensor puts out a smaller value for the air-fuel ratio (rich side) than the demand value as the altitude becomes higher, due to the characteristic of the sensor.
Since such atmospheric (exhaust gas) pressure dependence of the air-fuel ratio sensor increases the error in the sensor output, it is necessary to correct the sensor output depending on the altitude (elevation correction). Therefore, in the past, atmospheric pressure has been measured by using a special-purpose atmospheric pressure sensor (for example, Japanese Patent Application Laid-Open No. 1-159435 (1989), Japanese Patent Application Laid-Open No. 4-134152 (1992)).